All major web-based e-mail providers (Yahoo! Mail, Hotmail, GMail, AOL mail, etc.) give you the means to view the "full headers" of an e-mail message, and this functionality is also present in all major desktop e-mail programs like Microsoft Outlook.

By default, all of these programs and web-based e-mail services will only show you the "simple headers" of an e-mail message -- usually just the "From:" address, the subject line, and the date. The "full headers" can reveal more information about the transmission path that the message followed from the sender's machine to the recipient's server. If the message was sent from a user in the U.S., but they routed the outgoing message through a machine in China, and then the message arrived at your work e-mail address but was then automatically forwarded to your Yahoo Mail account, the full headers will reveal all of that type of information.

A description of how to read and understand the full headers of an e-mail message is beyond the scope of this short blog post. But if you want to view the full headers of an e-mail message in Yahoo Mail, it's easy.

Take for example this spam that I received at my Yahoo Mail account:

The "From" line says that it's sent from the address "AlfredRusso48@yahoo.co.uk". However, I suspect that that "From" address might be forged, and that the message did not really come from one of Yahoo's machines. So I scroll down to the bottom of the message, and in the lower-right corner is the "Full Headers" link:

Click on that, and now the message opens again, this time with the full headers displayed:

In this case I note that the last e-mail header (the last e-mail header chronologically, which is the first header displayed in the list), says:

Received: from 87.248.110.167 (HELO n34.bullet.mail.ukl.yahoo.com) (87.248.110.167) by mta359.mail.mud.yahoo.com with SMTP; Tue, 13 May 2008 15:50:02 -0700

To close the full headers, scroll down and click "Compact Headers" in the lower-right corner:

and the message will display with only simple headers, as before.

Bennett Haselton is a technology and political blogger who runs the world's largest proxy mailing list for helping people to get around Internet filters.